1. Field of the Invention
The present invention relates to a development device in which a dual-component developer is used and an image forming apparatus provided with the development device.
2. Description of the Related Art
In recent years, for electrophotographic image forming apparatuses supporting full-color and high-quality images, a dual-component developer (hereinafter simply referred to as the “developer”) which exhibits an excellent charge performance stability as to a toner is in widespread use. The developer is made up of the toner and a carrier, which are agitated in a development device and frictionally rubbed with each other to produce an appropriately electrified toner. In the development device, the electrified toner is supplied onto a surface of a development roller. The toner is moved by an electrostatic attraction from the development roller to an electrostatic latent image formed on a surface of a photosensitive drum. Thus, a toner image based on the electrostatic latent image is formed on the photosensitive drum.
Further, there has been an increasing demand for the image forming apparatuses that operate faster and that are miniaturized, which is associated with the necessity to electrify the developer quickly and sufficiently and to convey the developer quickly. To this end, Japanese Patent Publication Laid-Open No. 2001-255723 proposes a circulative development device including first and second developer conveying paths divided by a partitioning plate provided in a developer tank, first and second communicating paths that allow the first developer conveying path and the second developer conveying path to communicate with each other at opposite ends, and first and second auger screws that are arranged in the first and second developer conveying paths to convey a developer in directions opposite to each other.
In the development device described above, the developer in the first developer conveying path is conveyed by the first auger screw to an interior wall of the developer tank located at the most downstream side of the first developer conveying path, and is pushed toward the first communicating path due to a pressure from the developer conveyed from the upstream side, thereby transferring to the second developer conveying path. On the other hand, the developer in the second developer conveying path is conveyed by the second auger screw to the interior wall of the developer tank at the downstream side of the second developer conveying path, and is pushed toward the second communicating path due to a pressure of the developer conveyed from the upstream side, thereby transferring to the first developer conveying path. In this manner, the developer is circulated between the first developer conveying path and the second developer conveying path. Japanese Patent Publication Laid-Open No. 2009-109741 discloses a circulation type development device that differs from the circulation type development device of Japanese Patent Publication Laid-Open No. 2001-255723 in that an end on a downstream side in a developer conveying direction has an opposite spiral direction in a spiral blade of each of the first and second auger screws. In the development device of Japanese Patent Publication Laid-Open No. 2009-109741, the developer in the first developer conveying path is conveyed to the spiral blade having the opposite spiral direction of the first auger screw and pushed out on the first communicating path side by a pressure generated with the developer conveyed from the upstream side of the first developer conveying path, whereby the developer moves to the second developer conveying path. On the other hand, the developer in the second developer conveying path is conveyed to the spiral blade having the opposite spiral direction of the second auger screw and pushed out on the second communicating path side by a pressure generated with the developer conveyed from the upstream side of the second developer conveying path, whereby the developer moves to the first developer conveying path. Thus, the developer is circulated between the first developer conveying path and the second developer conveying path.
However, for the development devices disclosed in Japanese Patent Publication Laid-Open Nos. 2001-255723 and 2009-109741, the developer is subjected to a shear force in a compressed state while subjected to the rapid pressure in a travelling direction in a position in which the developer is located opposite the communicating path at a downstream end of the auger screw. A fluidity improver that is of a toner additive is buried in a resin particle constituting the toner by heat and the shear force, which are generated by a stress, which results in a phenomenon in which developer fluidity is extremely decreased to hardly convey the developer. As a result, the sufficient amount of developer is hardly supplied to the photoconductive drum through the development roller, and unfortunately density of an image printed in a recording medium is lowered.